It is well known that various cell hosts such as microbial cells, yeast cells, insects cells and mammalian cells are capable of producing small amounts (milligram quantities) of specific proteins in relatively short periods of time. Also, it is known that proteins from different genetically engineered host cell systems generate proteins having different glycosylation profiles, depending on the cell used. For pharmaceutical applications in some cases it is desirable to product proteins that have glycosylation profiles that are similar to those found in humans. This is usually accomplished using a mammalian cell system since such systems produce proteins with glycosylations that are compatible with clinical applications. The disadvantage of using mammalian cells is that they typically require a longer period of time to produce an equivalent quantity of protein. Thus, it would be very desirable to have a genetically engineered mammalian cell expression system (for expressing heterologous proteins) that can express larger amounts of the protein in shorter periods of time.
It is known that various episomal maintenance elements can be introduced into a vector in a gene expression system to ensure replication. It is also well known that genetically engineered systems can and in many cases should include strong promoter/enhancer systems. In addition, it is well known that genetically engineered cell lines can include protein transactivation systems to enhance protein expression from DNA coding for a heterologous protein that is also incorporated into the system. Against that background, we are unaware of any attempts or suggestions to incorporate all four of the above elements or systems into a single mammalian cell expression system so that the combined advantages of all of the elements are present in a single and functioning system that expresses heterologous proteins. To our surprise, when the above combination was made using the techniques described below we found what appears to be a synergistic effect on protein expression. Representative examples for two heterologous proteins (designated IL-2SA and IL-4SA) are described below.